1. Field of the Invention
The invention relates to a controller for use with photosensors to pick up the states of control elements, such as the keys of a player piano.
2. Related art of the invention
In general, a player piano has photosensors corresponding to its own control elements such as keys, the photosensors being controlled by the method of sequentially scanning, to detect the states of the control elements.
FIG. 5 shows a circuit schematic of a sensor unit of a photosensor controller. The driving circuit which is not indicated in FIG. 5 controls the turning on or off of all of the light emitting diodes (LED). A photodiode (PD) 2-1 receives the luminescence of the LED 1-1. A shutter 3-1 provided for a key corresponding to a note C# interrupts the luminescence of the LED 1-1 when the key is stricken by an operator.
Events of the key corresponding to the note C# are detected by the LED 1-1, the PD 2-1 and the shutter 3-1. Similarly, events of the key corresponding to a note D are detected by the LED 1-2, the PD 2-2 and the shutter 3-2, and the events of the key corresponding to a note D# are detected by the LED 1-3, the PD 2-3 and the shutter 3-3. Thus, each of 12 groups of an LED, a PD and a shutter is provided for detecting an event in the key for an octave.
Each of the PDs is connected to a power supply Vcc and a node A. The node A is directly connected to a negative input terminal of an operational amplifier 4, and is also connected to the ground via a resistor RL. The threshold voltage Vth is supplied to a positive input terminal of the amplifier 4. The output of the amplifier 4 is directly connected to a node B, and is also connected to the power supply Vcc through a resistor R lest the output be unstable.
If the level of the node A is higher than that of the threshold voltage Vth, the level of the node B will be the high voltage state ("H"). As shown in FIG. 5, the sensor unit 5 includes LEDs 1-1 to 1-12, PDs 2-1 to 2-12, shutters 3-1 to 3-12, an operational amplifier 4 and resistors.
Referring to FIG. 6, a timing chart of the sensor unit 5 is shown. As indicated in this drawing, the driving circuit supplies a periodic pulse signal one by one to the LEDs. For example, the LED 1-2 turns on as soon as the LED 2-1 turns off. Thus, the events of the keys are sequentially scanned.
For the purpose of explanation, the key depressed shall correspond to the note D. In other words, the luminescence of the LED 1-2 shall be interrupted by the shutter 3-2. In this case, at the luminescence timing of LED 1-1 and 1-3, which correspond respectively to PD 2-1 and 2-3, PD 2-1 and 2-3 receive the light from the respective LED. However, because PD 2-2 does not receive light at the luminescence timing of LED 1-2, the voltage level of the node A becomes "0" (low state) at the portion corresponding to the luminescence timing of LED 1-2, as is shown in FIG. 5. Through a comparison of the level of the node A and the threshold voltage Vth by the operational amplifier 4, output signal B is obtained.
The output signal B is sampled by sampling pulses which are placed at center of the luminescence timings. Each of sampling pulses is short in comparison with the luminescence timing. Thus, an event in the key corresponding to the note D is detected by a comparison between the sampling results and the luminescence timing of the LEDs.
One problem associated with LED or PD, is that considerable variation in characteristics between devices exist. If, for example, variation in the luminescence energy between the LEDs exists, a difference is generated in the potential between the cathodes of the PDs. Moreover, this difference in potential is also generated by the light receiving characteristics of the PDs themselves.
By the time constant constituted from the load resistor RL and the junction capacitor of the PD itself, the rise and fall portions of the waveform at the node A, as indicated in FIG. 6, come to round. When rounding becomes severe, because, while the timing in comparison to the output at node A is delayed by delta t, the timing attains the voltage threshold value Vth. This characteristic of delay determines delay time constant. It is still possible to detect the events of keys by supplying the sampling pulse after the elapse of delta t.
However, if differences between output voltages at the PDs exist due to the aforementioned variation in the PDs, another problem referred to as "cross talk" arises. In other words, when continuous keys are in "OFF" state, for example when neighboring shutters such as shutters 3-2 and 3-3 dont't block the light of LEDs 1-2 and 1-3, a step differential is generated at the output level of node A (omitted from the figures), causing a mutually deterrent effect.
The delay delta t, cannot be ignored to the extent that the speed (frequency) of the scanning of the keys increases. In the worst case, before the elapse of the delay delta t, the next key is scanned, and thereby, the output B cannot follow the scanning speed and does not change. Therefore, in this case, it is impossible to detect key events.
One idea associated with the improvement of the delta t is that an LED of the high luminous efficacy type or a PD of the high speed response type is used to detect the event. However, problems associated with this idea include the fact that such an LED or a PD is very expensive and that, if a large current flows through such devices, the reliability and the endurance of the devices decrease.